Old bench grinder what is this part?

Thread Starter

themow

Joined Dec 28, 2012
17
I was given a heavy old bench grinder 110v ac motor with shaft for a wheel on each side. The motor plate is missing so I dont know anything about the motor.

Its plug was missing so I shoved the bare wires into an outlet to test and it powered on fine.
I put a plug onto it and it powered on but sounded different and then started smoking and made a pop sound. I unplugged it and waited an hour and plugged it back in and its just making a buzzing sound.

The only thing under the hood is a switch and this part in the pictures below. Is the polarity important on this setup?

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ian field

Joined Oct 27, 2012
6,539
That appears to be the current relay for switching the Start capacitor, as soon as it is up to run, it switches the cap out, otherwise it will sit there and Hum, if you were to give the wheel a sharp spin, it would normally run.
Polarity is not important .
http://www.achrnews.com/articles/92424-know-your-potential-starting-relays
Max.
Some types of motor don't have a start capacitor - you just power the start winding until it spins up, any longer lets the magic smoke out though!

The relay is a current relay, the motor in the stalled condition draws enough current to pull the relay in, as it spins up the relay drops out.

Some manufacturers took to putting a PTC thermistor in series with the start winding - once or twice I've got away with bodging them with a TV degauss posistor.

If the motor uses a start capacitor - the PTC method might not work.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
The op says it is an older unit, so if it had not been ran for some considerable time, the bearing may have partially seized or become corroded, and if it is the non-capacitor start motor, they require zero load to start, if any considerable resistance they can burn out.
Max.
 

ian field

Joined Oct 27, 2012
6,539
The op says it is an older unit, so if it had not been ran for some considerable time, the bearing may have partially seized or become corroded, and if it is the non-capacitor start motor, they require zero load to start, if any considerable resistance they can burn out.
Max.
They usually buzz when that happens - most people would give it a shove to see if it starts going, and/or unplug it as the magic smoke starts billowing.

They usually have impeller blades on one end of the rotor for what little air flow they shift. Failure to open the start contacts on an otherwise running motor may not release all that much magic smoke all at once - the smoke could even go un noticed.
 

Thread Starter

themow

Joined Dec 28, 2012
17
Its alive!!!!
I bypassed the start relay and wired the hot directly to the run winding. As someone mentioned in a previous post, there were 3 windings in this motor. neutral, hot, and start.
I wrapped a rope on the shaft and turned it on and gave it a pull and she came right to life!!!
Ran it for 5 minutes and is working fine.
I hooked up a 250v 10uf capacitor hoping that would get it going but it didnt budge.
Its probably a 1/3hp 3450rpm motor.
Will the proper sized cap work? If not I may just stick to the rope method. THis motor is super smooth.

What I think initially happened was that the relay was pushed up against the base and forced constant contact.
 

ian field

Joined Oct 27, 2012
6,539
Its alive!!!!
I bypassed the start relay and wired the hot directly to the run winding. As someone mentioned in a previous post, there were 3 windings in this motor. neutral, hot, and start.
I wrapped a rope on the shaft and turned it on and gave it a pull and she came right to life!!!
Ran it for 5 minutes and is working fine.
I hooked up a 250v 10uf capacitor hoping that would get it going but it didnt budge.
Its probably a 1/3hp 3450rpm motor.
Will the proper sized cap work? If not I may just stick to the rope method. THis motor is super smooth.

What I think initially happened was that the relay was pushed up against the base and forced constant contact.
You could probably flick it round enough with your fingers - but if you forget; the run winding will burn out.

As I mentioned in an earlier post - some manufactures just put a PTC thermistor in series with the start winding, once or twice I've got away with a PTC element from a CRT TV degauss unit.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
Its alive!!!!
I bypassed the start relay and wired the hot directly to the run winding.
I wrapped a rope on the shaft and turned it on and gave it a pull and she came right to life!!!
Ran it for 5 minutes and is working fine.
I hooked up a 250v 10uf capacitor hoping that would get it going but it didnt budge.
Its probably a 1/3hp 3450rpm motor.
.
If by passing the switch, make sure you do not power the start winding, if left connected it could burn out the start winding, if not cooked already, if you fitted a manual switch in place of the current relay then you could do it this way as long as you switch it off as just before it comes up to speed, IOW a couple of seconds at best, as long as the start winding is still OK.
It would be a 2 pole motor for a bench grinder and fractional HP if no capacitor.
Max.
 
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Thread Starter

themow

Joined Dec 28, 2012
17
I guess I was not clear in my last post.
The start winding is cooked.
The start relay works, but since the start winding is cooked, I disconnected it.
Now power only goes to the 2 main windings

In order to get up to full speed, spinning by hand is no good. Need to wrap a strap around the shaft and give it a good pull. Then flip the switch.
After that, she purrs

Can I add a capactior to fix this problem? If not, what about a ptc?
I tried a cap from my other bench grinder but it wouldn't budge. 250v 10uf
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
If the start winding is cooked, then a cap is not going to be of any use, as you would normally put the cap in series with the start winding, but normally there is already some phase shift in this start winding to begin with so that would not be a success, for the price of bench grinders now, it would not be worth getting it rewound.
Max.
 

ian field

Joined Oct 27, 2012
6,539
If the start winding is cooked, then a cap is not going to be of any use, as you would normally put the cap in series with the start winding, but normally there is already some phase shift in this start winding to begin with so that would not be a success, for the price of bench grinders now, it would not be worth getting it rewound.
Max.
There is/was a rewind firm near me.

The quote they gave me for an angle grinder was more than the price of a much better quality tool than the one I wanted to repair.
 

Thread Starter

themow

Joined Dec 28, 2012
17
OK I didn't know motors with caps had start windings. Thank you guys for all your help. I'm going to use the rope start method for a while becasue newer Chinese grinders are no where as smooth. Thank you all
 
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